A Laboratory Manual of the Anatomy of the Rat/The Digestive System
THE DIGESTIVE SYSTEM
The digestive system consists of the mouth cavity, pharynx, esophagus, stomach, small and large intestines, salivary glands, liver and pancreas.
The mouth opening is triangular. It is located on the anteroventral surface of the head some distance back from the tip of the snout. The lower lip conceals the greater part of the two long lower incisor teeth. Externally it is hairy; internally it is covered with smooth mucous membrane. A fleshy ridge on its inner surface fits into the space between the incisor teeth. This ridge joins ventrally a short vertical septum, the frenulum, which passes between the incisor teeth, connecting the lip with the mandible. From the angle of the mouth a lobe of integument, covered with hair, turns upward along the inner surface of the cheek.
The lateral halves of the upper lip meet in front at an acute angle pointed toward the nasal septum. The apex of the angle is connected with the septum by a groove. The upper lip is therefore cleft, exposing the incisor teeth of the upper jaw. A frenulum is lacking. The edge of each upper lip immediately behind the incisor teeth is turned inward, forming a lobe whose lower hairy surface touches the dorsal surface of the tongue. This lobe is hairless dorsally, and is in contact here with the roof of the mouth. The lip lobes, tongue, and a fleshy tubercle on the middorsal surface of the mouth's roof just behind the lobes, are all in contact with one another when the jaw is closed, practically separating the mouth opening from the mouth cavity.
The teeth have been described in connection with the bones of the cranium. There are two long pairs of chisel-like incisor teeth, one pair in the upper and one pair in the lower jaw. The lower pair protrudes farther into the mouth cavity than the upper. The upper incisors incline slightly backward. The lower extend forward from the anterior end of the under jaw, curving upward to their points which lie a little behind the tips of the upper incisors when the jaw is closed. Immediately behind the incisor teeth is the toothless region, the diastema. The three upper and three lower molar teeth for grinding the food torn loose by the incisors, are back of the disastema.
The mouth cavity comprises two parts, the vestibule lying between the teeth on the one hand and the lips and cheeks on the other, and the mouth cavity proper. The vestibule is composed of two completely separated parts—the incisor and the molar. The former lies between the incisor teeth and the lips. On the lower jaw it is well developed, but on the upper is almost completely lacking on account of the cleft in the upper lip. The molar region of the vestibule is a narrow region between the molar teeth and the cheek. The discontinuity of the two parts of the vestibule is due to the absence of teeth in the diastema. The folds of the upper and lower lips and cheek are the only structures on the lateral side of the mouth in the diastema region. The mouth cavity proper is widest in the region of the molar teeth. It decreases in width anteriorly and between the tongue and soft palate extends back as a narrowed passage opening into the pharynx through the isthmus faucium at the posterior end of the soft palate.
The roof of the mouth comprises two regions, the hard palate supported by the palatine processes of the premaxillary, maxillary, and palatine bones, and the soft palate, which consists of a boneless, flexible wall extending backward from the hard palate. The surface of the soft palate bears small papillae. The palate is widest at the posterior part of the molar tooth region, decreasing in width forward to the inturned lobes of the upper lips, and backward to the pharynx. A membranous ridge, the palatine arch (pillar of the fauces), extends ventrally from the posterior end of the soft palate on each side of the isthmus faucium. The hard palate bears eight transverse ridges, the most anterior of which is ventral to the prepalatine foramina. The three anterior ridges are the largest and lie in front of the molar teeth. The other five ridges are within and behind the molar region. These five may assume approximately the form of a letter w, with the base of the letter forward, or the form of a bow, or of an obtuse angle whose anteriorly directed apex lies in the mid-dorsal region of the palate. Each of the five ridges bears a row of short spines throughout its length. Other denticulated ridges, which do not reach the median line of the hard palate, may also be present. At the posterior end of the hard palate is a triangular area bearing spines.
Carefully disarticulate the lower jaw. The tongue is an elongated muscular organ extending from the pharynx forward to the incisor teeth. It fits snugly into the floor of the mouth between the molar teeth. Anterior to the molars it is unattached to the mouth floor. The vertical frenulum which in some animals connects this anterior free region of the tongue with the mouth floor is absent. A median dorsal furrow extends seven or eight millimeters back from the tip. The tongue is attached to the floor of the mouth from the anterior molar teeth caudad. The proximal one-sixth of the organ is attached on both sides also, so that it alone forms the floor at the back of the mouth. The tongue unites with the hyoid bone a short distance in front of the epiglottis. Papillae cover the dorsal surface of the anterior three- quarters of the tongue, the tip, and dorsal portions of the lateral surfaces. They are entirely lacking on the posterior quarter of the upper surface, ventral portions of the lateral surfaces, and the under side of the free anterior end. The filiform papillae are slender pointed structures with their tips generally directed backward. On the anterior half of the dorsal surface these papillae are rather conspicuous when seen under the dissecting microscope. In a triangular area about half way between the anterior and posterior ends of the tongue the filiform papillae are larger than elsewhere, conical and erect. The filiform papillae back of this area are so numerous and slender that here the tongue has a velvety appearance under the dissecting microscope. A few, low, cylindrical fungiform papillae are scattered over the anterior half of the dorsal surface, chiefly toward the tip.
There is no organized tonsil. This organ is represented merely by scattered follicular openings on the longitudinally folded side walls of the narrow faucial canal.
Exercise XX. Sketch a dorsal view of the tongue.
The soft palate forms the ventral floor of the nasopharynx (pars nasalis). This is a relatively large cavity which opens anteriorly through the single posterior naris into the nasal cavity. Toward its posterior end the nasopharynx curves downward and opens into the pharynx proper. The lateral walls are supported by the sphenoid processes of the palatine bone and the median pterygoid processes of the alisphenoid, while the basisphenoid and presphenoid bones support the roof. The auditory (Eustachian) tubes from the middle ears open into the nasopharynx about half a centimeter from the posterior border of the soft palate. The opening of each tube is a longitudinal slit near the angle between the lateral and dorsal walls, on the same side of the nasopharynx as the ear with which the tube is connected.
The pharynx proper lies back of the mouth cavity. It has four openings. Anterodorsally it communicates with the nasopharynx, anteroventrally with the mouth cavity, through the isthmus faucium, posteriorly with the esophagus, and ventrally with the larynx and trachea through the glottis, the entrance to the larynx. The food coming from the mouth passes through the pharynx to the esophagus. The air entering and leaving the lungs also traverses the pharynx. It is therefore a part both of the digestive and respiratory tracts. During respiration the free end of the epiglottis is raised to allow the passage of air to and from the larynx. The epiglottis bends backward and covers the glottis when food is swallowed, preventing the entrance of food into the trachea. A u-shaped, fleshy ridge on the dorsal wall of the pharynx, together with the posterior edge of the soft palate, form an ovoid opening into the nasopharynx which incloses the epiglottis and the glottis when the roof of the pharynx is depressed. Probably this arrangement permits breathing to continue, without drawing particles into the respiratory passages, while the animal is gnawing materials which it does not swallow. A recess of considerable size lies lateral to this semicircular ridge.
The esophagus is the tube which conveys food from the pharynx to the stomach. From its position anteriorly on the dorsal side of the larynx, it follows the dorsosinistral surface of the trachea to the level of the first rib, where it rests against the left surface of the trachea. Entering the thorax, the esophagus runs backward, dorsal to the aortic arch and primary bronchus, to the diaphragm, which it pierces. Posterior to the liver, in the abdominal cavity, the esophagus enters the stomach. Open the abdominal cavity by a median incision from the diaphragm to the pelvis.
The stomach lies on the left side in the abdominal cavity, in contact with the concave posterior side of the liver. It resembles a half of a doughnut in shape. The long axis forms approximately a right angle with the long axis of the body. The greater curvature which corresponds to the outer edge of the doughnut, is ventral. The lesser curvature, analogous to the rim of the doughnut's hole, is dorsal. The esophagus enters the stomach on its dorsal side, through the cardiac orifice, or cardia, near the center of the lesser curvature. The esophagus is closer to the right half of the lesser curvature than to the left, thus entering the stomach obliquely. External inspection of a full stomach reveals two distinct regions. The wall of somewhat more than the left half, the cardiac sac, is flexible, thin, and in a preserved specimen sufficiently transparent to permit the food it contains to be seen from the outside. The wall of the right region of the stomach, on the other hand, is opaque and relatively firm. This part decreases in size dorsally and communicates with the small intestine through the pylorus. The inner surface of the stomach comprises two areas corresponding to the two regions seen from the outside. These may be seen to good advantage by opening the distended organ with an incision extending throughout the length of the greater curvature. The inner surface of the cardiac sac at the left is hard, smooth, and glossy. The wall is tough. If the stomach was distended with food when the animal died, the wall is slightly transparent, the transparency increasing as the tissue dries. In the region at the right the inner surface is yellowish in color, opaque, brittle in a preserved animal, and more or less prominently folded, especially near the pylorus and on the greater curvature. The two re gions are sharply separated by the horny boundary fold, which extends ventrodextrad, on the anterior and posterior surfaces, from the vicinity of the cardia. The wall of the cardiac sac is opaque in an empty stomach, and its inner surface is extensively folded. The esophagus opens into the cardiac sac, for it lies at the base of the tongue-like dextral extension of the sac's horny lining. The part of the boundary fold bordering the right end of this extension is highly sinuous.
Food passes to the small intestine through the pylorus, at the upper right hand extremity of the stomach. The pylorus contains a ring of muscle whose contraction closes the digestive tube, shutting off communication between the stomach and intestine. It regulates in this manner the passage of food from the stomach to intestine. The position of the pylorus is indicated externally by a groove around the alimentary canal. Internally the high ridges in the glandular part of the stomach are replaced at, or slightly to the right of, the pylorus by the velvety surface of the small intestine.
The stomach wall of the mouse (and rat also) are said to show the following structures. There are two muscle layers, a thick circular and a thinner longitudinal layer. The cardiac sac and boundary fold are lined internally by the statum corneum, or horny layer of epithelial cells. There are three zones in the glandular tissue at the right of the boundary fold. The cardial glands surround the tongue-like extension of the horny epithelium at the right of the cardia, and extend ventrally along the anterior and posterior walls. The rennet glands are on the ventral wall at the right of the boundary fold. The pyloric gland region lies between the pylorus, and the cardial and rennet glands. The glands of the mammalian stomach secrete the gastric juice which initiates the digestion of protein substances in the food. The cardiac sac of the rodent, with its horny lining, is thought to be a storehouse for holding chewed food pending gastric digestion. It has been suggested that the dangers involved in feeding have led Rodents to develop this thin-walled sac in which quickly procured food may be stored.
The intestine is the part of the alimentary canal in which the digestion and absorption of food is completed. It is an extensively coiled tube occupying a large part of the abdominal cavity. It is physiologically and morphologically divisible into two parts, the small and the large intestine. The discussion of the mesenterial supports will be deferred until after the description of the alimentary canal itself. The length of the small intestine may be about 70 centimeters, more than three times the length of the animal from the tip of the snout to the base of the tail. The large intestine, including caecum and rectum, is about 20 centimeters long. The small intestine comprises the duodenum, jejunum, and ileum.
The duodenum lies on the right side of the abdominal cavity. It is approximately u-shaped, and is therefore composed of three parts — a descending, a transverse, and an ascending limb. The first joins the stomach at the pylorus. From this attachment it turns posterodextrad in a broad curve, and runs backward about four centimeters near the right abdominal wall. The transverse duodenum is the sharp short curve at the posterior end of the descending limb, connecting the latter with the ascending limb. The latter passes forward along the right side of the colon, and is connected by a mesentery with the distal edge of the mesocolon. lii a preserved specimen the duodenum is pinkish or slightly grayish in color. The gross structure of the internal surface may be observed to good advantage with a dissecting microscope if a fresh duodenum be slit lengthwise, spread out, thoroughly cleaned, and hardened with formalin. The mucous membrane is folded into very closely packed, relatively short, flat, transverse ridges. The longer ridges follow a zig-zag course and their distal edges are lobed. The ridges probably facilitate digestion by increasing the secretory and absorptive surface of the intestine.
The jejunum follows the duodenum. It constitutes in man two-fifths of the small intestine, exclusive of the duodenum. Its inner surface, like that of the duodenum, is thickly covered with short, flat, transverse ridges.
The ileum is not sharply separated from the jejunum in man. It connects the jejunum and the colon. In the rat it is characterized by its dark color, due to the color of the contained feces. The mucous membrane of the ileum is also thrown into numerous closely packed transverse ridges.
The contents of the ileum pass through the ileocolic valve into a small chamber which communicates by wide openings both with the colon and the caecum. The latter is a sac measuring about two and one-half centimeters on its longest axis. It varies in shape, sometimes resembling an interrogation mark, or assuming the form of a short spiral. Thus it has a greater, or outer, and a lesser, or inner, curvature. At one end it opens into the intestines. A mesentery connects the lesser curvature with the ileum. The caecum contains a mass of dark fecal matter. When this is removed it will be seen that the inner surface presents smooth areas of considerable extent, but bears folds also. These folds are transverse along the inner curvature, but near the proximal end they radiate, in general, from the opening of the caecum.
From the caecum and ileum the feces enter the ascending colon. This follows an irregular route forward, ventral to the duodenum but dorsal to loops of the jejunum and ileum, to the region of the stomach. Here the colon (transverse colon) circles dorsally and to the left as far as the sagittal plane of the body. The descending colon follows approximately the sagittal plane of the body back to the rectum. The ascending limb of the duodenum lies at the right of the descending colon, and both are close to the dorsal surface of the abdominal cavity. The rectum is the continuation of the colon through the pelvis. It terminates posteriorly at the anus where the feces are expelled. There is no sharp boundary between colon and rectum. The dark fecal matter in the colon forms pellets shaped like cotton seeds. Wherever these masses occur the inner surface of the colon is smooth. Elsewhere the lumen is practically devoid of feces and the surface is thrown into prominent folds, the colic ridges. The absence of ridges and the greater diameter of the colon where the fecal pellets occur shows that at least one purpose of the ridges is to permit distension of the mucous membrane of the colon without undue strain upon it.
The liver is a firm, bulky, dark red organ lying in the anterior region of the abdominal cavity. Its convex, cranial surface rests against the concave posterior face of the diaphragm. Ventrally it rests upon the abdominal wall. Its caudal surface is concave and is in contact with the stomach and intestines. The organ is deeply cleft into several very distinct lobes and lobules.
The median lobe occupies a median position in front of the other lobes of the liver. A longitudinal fissure near the sagittal plane of the body partly divides this into two lateral lobules, approximately equal in size, sometimes referred to as the right and left central lobes. This fissure extends from the thin posterior edge of the lobe nearly to the ventral border of the diaphragm, where it receives the round ligament. Dorsal to this point the two lobules are united anteriorly, and are in contact with the diaphragm. Posteriorly, the fissure extends upward, sometimes reaching the region of the inferior vena cava. The right lobule passes upward to the right of this vein, the left lobule to the left. The median lobe is fastened in this region to the vena cava. Very little, if any, hepatic glandular tissue connecting this lobe with the rest of the liver, is visible in a gross dissection.
The left lateral lobe is immediately behind and at the left of the median lobe of the liver. It attaches to the other lobes of the liver only at its dorsomedial angle, and here merely by its ducts, blood vessels, and connective tissue. The anterior surface is strongly convex, the posterior surface is concave and rests against the stomach and intestines. The lobe is thick near the center, tapering to a thin edge peripherally. The right side pushes in between the median and right lateral lobes.
The right lateral lobe lies behind and at the right of the median lobe. A vertical transverse cleft divides it into two almost completely separated lobules, one anterior, the other posterior. A bridge of hepatic tissue crossing the dorsal side of the inferior vena cava connects the two lobules with one another and with the caudate lobe on the left side of the abdomen. The posterior lobule is frequently annexed to the caudate lobe by a strip of hepatic glandular tissue crossing the ventral surface also of the inferior vena cava. Both lobules attach to the right surface of the vena cava. Aside from the glandular bridges mentioned above, a gross dissection fails to reveal connections of hepatic tissue with the other lobes of the liver. The anterior lobule is thick near the center, thinning out toward the sharp peripheral edges. Its convex anterior surface fits into a concavity near the right border of the median lobe. The right lateral surface rests against the diaphragm. The caudal surface presents a ventral convexity and a dorsal concavity to the cranial surface of the posterior lobule. The concavity receives also the anterior surface of the right kidney and the accompanying adrenal gland. A ventral strip of the posterior surface is presented to the descending limb of the duodenum. The posterior lobule is approximately pyramidal. The base of the pyramid fastens to the right side of the inferior vena cava. The triangular surface facing the anterior lobule is slightly concave medially, decidedly convex laterally, so that the apex of the pyramid points backward. The renal impression is the concave triangular dorsal surface which receives the ventral side of the right kidney. A spur of the lobule runs dorsal in front of the kidney. Occasionally the posterior lobule is more or less divided into two secondary lobules.
The caudate lobe (Spigelian lobe) comprises two distinct semicircular lobules, the anterior and the posterior lobules. These unite mediad with each other and with the left side of the inferior vena cava, but are separated laterally. As previously mentioned, the caudate lobe is annexed to the right lateral lobe dorsal, and sometimes ventral, to the inferior vena cava. These lobules fit neatly into the lesser curvature, and adjacent surfaces, of the stomach. The anterior lobule is in front of this organ, the posterior behind it, lying in the lesser peritoneal cavity. The esophagus fits into a groove, the esophageal impression, on the caudal surface of the anterior lobule.
The liver and diaphragm are united by several membranous ligaments which radiate outward from the region where the inferior vena cava pierces the diaphragm. The falciform (or suspensory) ligament of the liver lies in the sagittal plane of the body. It connects the diaphragm ventral to the inferior vena cava with the anterior surface of the median lobe. The round ligament (ligamentum teres hepatis) is the ventral thickened border of the falciform ligament. It enters the dorsal end of the fissure in the median lobe. The left triangular ligament is a triangular sheet connecting the dorsal edge of the left lateral lobe with the diaphragm. It unites with the diaphragm along a line running from the vena cava to the left and to the dorsal margin of the diaphragm. The right triangular ligament is a comparatively broad sheet connecting the dorsal side of the right lateral lobe with the diaphragm.
The rat's liver possesses no gall bladder for storing the bile, the digestive secretion of the liver. The hepatic ducts convey the bile from the liver lobes to the bile duct (ductus choledochus) which then passes along the duodenohepatic omentum, ventral to the portal vein. The duct traverses the dorsal side of the pancreas and enters the mensenterial side of the duodenum about three centimeters from the pylorus. The liver receives arterial blood from the hepatic artery, and venous blood from the alimentary canal through the portal vein. Food substances absorbed from the alimentary canal are carried to the liver through this vein. Carbohydrates are stored there in the form of glycogen. Blood leaving the liver enters the inferior vena cava through the hepatic veins.
MESENTERIES OF THE ALIMENTARY CANAL
The digestive organs of the vertebrates are connected with one another and with the walls of the abdominal cavity by membranes. The membranes uniting the organs with the dorsal abdominal walls are called mesenteries. Of these the mesogastrium supports the stomach, the mesentery proper the intestine, and the mesocolon supports the colon. The alimentary canal in all vertebrates except the Cyclostomes is more or less convoluted, and the parts are held together by mesentery-like structures. The hepatogastric ligament ties the stomach to the liver, the gastrosplenic ligament connects stomach and spleen, the hepatoduodenal ligament unites duodenum and liver.
Though the mesenteries of the rat have doubtless evolved from a simpler arrangement, such as that found in the dogfish, they are relatively complex, and require careful study.
The mesogastrium of the rat is expanded into the form of a sac, the greater omentum, which projects backward from the stomach region. The rim of this sac unites with the stomach upon a line extending from the pylorus along the greater curvature of the stomach, around the left end and back to the esophagus on the lesser curvature. Dorsad the greater omentum unites with the body wall between the adrenal glands. The omental sac thus comprises two limbs, a dorsal one united anteriorly with the dorsal abdominal wall, and a ventral limb attached to the greater curvature of the stomach.
The spleen is suspended along its median surface to the left side of the omental sac. It lies parallel to the greater curvature of the stomach. The part of the great omentum connecting the spleen and stomach is the gastrosplenic or gastrolienal ligament. A part of the pancreas is lodged in the descending limb of the great omentum.
The descending colon lies in the sagittal plane of the body. It is held in this position throughout its length by the relatively narrow mesocolon, which joins the dorsal abdominal wall just beneath the inferior vena cava. The mesocolon terminates about three and one-half centimeters from the anus. Anteriorly it fastens on the left to the great omentum. The duodenocolic omentum connects the descending colon and the ascending limb of the duodenum. Its attachment extends throughout nearly the whole ascending limb of the duodenum. Its connection with the descending colon is in common with that of the mesocolon.
The mesoduodenum spans the gap between the ascending and descending limbs of the duodenum. It joins the dorsal abdominal wall at the anterior end of the duodenal loop, as described below.
At its anterior end the duodenum turns sharply ventrad to join the jejunum. The mesentery of the jejunum and ileum serves also for the ascending colon, and is therefore both mesentery of the small intestine and a part of the mesocolon. The central axis of the mesentery carries the blood vessels which bear blood to and away from the intestines. The jejunum and ileum are very much longer than the axis of the mesentery. Therefore the mesentery's edge attached to them is expanded into many convolutions. This arrangement gives great freedom for movement of the small intestines, so that the peristaltic action, which forces the food along the canal, is not hindered by the mesenterial attachments. If an anaesthetized animal is opened, the movements of the small intestines can be seen.
The ascending colon is about as long as the axial portion of the mesentery. Therefore this edge of the mesocolon is not thrown into folds. A short triangular mesentery passes between the caecum and the distal end of the ileum. The transverse colon is tied to the mesoduodenum by a short mesentery. The length of the ascending mesocolon, as compared with the relatively short mesenterial connections of the transverse and descending colons, leaves the ascending colon relatively free for movement. Proximally this complex of jejunal and ilial mesenteries and mesocolon, fuses with the anterior part of the mesoduodenum. The combined mesenteries then join the dorsal wall of the ab domen between the kidneys and at the left of the inferior vena cava.
The pancreas secretes a digestive fluid, the pancreatic juice, which contains three enzymes, trypsin, steapsin, and amylopsin. The gastric juice of the stomach contains pepsin which reduces protein food substances to soluble peptones. These, in turn, the trypsin breaks down into amino acids, which are absorbed in the intestines. Steapsin decomposes fats into fatty acids and glycerin. Amylopsin reduces all digestible carbohydrates to glucose, which is absorbed and stored in the liver as glycogen.
The pancreas of the rat is irregular in shape and divided into many large and small lobes. Some of these lobes anastomose with each other. The pancreas is suspended in the descending limb of the great omentum, and throughout the mesoduodenum. It bends dorsally in the mesen- terial junction of these membranes, and also sends a lobe to the pyloric region of the stomach. Its left end is dorsal to the spleen. Pancreatic tissue lies close against the mesenterial side of the duodenum throughout the duodenal loop.
The pancreatic juice enters the bile duct, as it crosses the pancreas, through several small ducts. The openings of these ducts into the bile duct may be seen if the latter is slit and its inner surface examined under the dissecting microscope. The bile duct conducts the pancreatic juice, with the bile, to the duodenum. These ducts are identified with difficulty unless a careful dissection be executed using a microscope.
Exercise XXI. Make an enlarged drawing of the opened abdominal cavity, showing as many as possible of the contained organs and mesenteries.